The present invention relates generally to apparatus for delivering a measuring or detecting probe to a work position and, in particular, to apparatus for delivering a probe which is mounted on an elongated, flexible carrier.
The invention has particular application to the maintenance of a steam generator, particularly a nuclear power plant steam generator.
A typical nuclear steam generator comprises a vertically oriented vessel, a plurality of U-shaped tubes disposed in the vessel so as to form a U-shaped tube bundle, and a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at the one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature, downwardly through second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam.
Isolation of the radioactive primary fluid from the secondary fluid in a nuclear steam generator is critical. Accordingly, test procedures have been developed to test the integrity of the generator tubes. One such procedure is eddy current inspection of the steam generator tubes. In this inspection process, an eddy current probe must enter and travel through a steam generator tube for approximately 60 feet and be retracted at a constant rate. The probe is mounted at the end of an elongated flexible carrier conduit through which probe conductors extend to control and recording apparatus.
The probe is fed into the tube bundle by a probe pusher which consists of a mechanical gear train driving a series of drive rollers which frictionally engage the probe carrier. These frictional rollers impart linear movement to the probe carrier and provide a slippage cluctching action in the event that the probe contacts an obstruction which impedes its passage. Thus, the frictional driving force of the drive rollers is set so that slippage will occur in the event of an obstruction so as to prevent possible damage to the probe. Retraction of the probe is accomplished by a take-up drum which is mechanically driven with a mechanical slippage clutch. Thus, in the event that the probe should become stuck upon retraction, the clutch will slip to prevent damage to the carrier and/or the probe.
A problem with this type of probe user is the frictional wear on the frictional drive rollers as a result of the slippage between the rollers and the probe carrier. This necessitates frequent replacement of the rollers. Furthermore, the probe pusher apparatus is a rather complicated mechanical device utilizing a large number of gears, bearings, ratchet drives, sprockets and the like, which results in large physical size and weight of the apparatus, complicated and expensive construction and expensive and time consuming maintenance.
Another disadvantage of that prior probe pusher apparatus is that adjustment of the tension on the roller drive must be manually effected. Thus, for example, if the probe becomes stuck so that slippage occurs in the clutch mechanism, it must be manually adjusted to apply the proper force to effect withdrawal of the probe. Also, as the clutch rollers wear, they must be periodically adjusted to maintain the proper frictional driving force. Each time such an adjustment or a parts replacement or other maintenance is performed on the apparatus, it results in exposure of maintenance personnel to radioactivity and loss of operating time. The maintenance and adjustment problem is so severe that, typically, on eddy current inspection field trips a repair person is on duty for no other reason than eddy current probe pusher maintenance.
Another drawback of the prior probe pusher apparatus is that the slippage between the drive rollers and the probe carrier prevents the measurement of probe movement by measuring the movement of the drive rollers. Thus, separate means must be provided for accurately determining the position of the probe.